1. Field of the Invention
The present invention relates to underwater concrete and more particularly to a method of placing concrete underwater comprising pumping a fluid cementitious mixture wherein the rheology, flow characteristics and washout resistance of the cementitious mixture are readily controllable at the point of discharge.
2. Description of the Related Art
The placing of concrete underwater, whether for repair of existing structures or the building of new structures, often results in the washout of fines from the freshly mixed concrete and/or in segregation of the concrete mix. In order to prevent this, most concrete mixes used for underwater placement include an anti-washout admixture. Most of the anti-washout admixtures typically include a cellulose material to increase the water retentivity and thixotropy of the mixture so that the mixture is fluid enough to be transported under water, e.g. by pumping, but sufficiently cohesive to minimize segregation and washout. For many underwater applications, one or another of the cellulose based anti-washout admixtures is sufficient. However, in situations where large amounts of concrete need to be placed in areas where there is severe water flow and/or water pressure, such as repair of dams and back filling the annular void behind tunnel linings, the cellulose based anti-washout admixtures can be insufficient in preventing washout and segregation.
Thus, there is a need for a method of placing concrete underwater wherein the fresh cementitious mixture is fluid enough to be pumped yet sufficiently cohesive once it reaches water to prevent segregation and washout even under conditions of severe water flow and/or water pressure.